1. Field of the Invention
This invention relates to a photo-addressable type recording display apparatus, and more particularly relates to a photo-addressable type recording display apparatus provided with a light switching element and a display element combined together.
2. Description of the Related Art
Recently, a photo-addressable type spatial light modulation device provided with functional elements such as photoconductive light switching element and a display element that are combined together has been developed, and has been used practically as a light valve of a projector. Furthermore, it is tried to use it in the optical information processing field as described in xe2x80x9cLiquid crystal spatial light modulator and information processingxe2x80x9d EKISHO, Vol. 2, No. 1, 1998, pp. 3-18.
Furthermore, for example, as described in H. Yoshida, T. Takizawa, et al., xe2x80x9cReflective Display with Photoconductive Layer and a Bistable, Reflective Cholesteric Mixturexe2x80x9d SID, 1996, APPLICATION DIGEST, pp. 59, a recording display medium provided with a display element formed of cholesteric liquid crystal and a light switching element formed of amorphous silicon and Elgraphy system that is an image input system formed of laminated memory liquid crystal and organic photosensitive material have been studied as a photo-addressable type recording display medium.
As the display material used for the display element of such photo-addressable type recording display medium, various liquid crystal material such as nematic liquid crystal, twist nematic liquid crystal, super twist liquid crystal, and smectic liquid crystal, surface-stabilized ferroelectric liquid crystal, polymer dispersion liquid crystal that is formed by polymer-dispersing the liquid crystal material as described hereinabove, and capsulated liquid crystal that is formed by capsulating the liquid crystal material as described hereinabove have been known.
A photo-addressable type recording display medium provided with a display element formed of memory display material is capable of holding the display state without electric power for holding the recording display. Therefore, because the display state is held even if the display medium is detached from a writing apparatus after the digital information has been recorded and displayed, it is possible to carry away the display medium that has been detached from the writing apparatus, and such display medium has been attract much attentions as an electronic paper medium.
For example, cholesteric liquid crystal, ferroelectric liquid crystal, polymer dispersion liquid crystal, and capsulated liquid crystal have been known as memory display material, and application of these display material to a recording display medium that is detachable from a power source has been studied.
Particularly, a filter used for color selection is unnecessary for cholesteric liquid crystal or cholesteric capsulated liquid crystal that is formed by capsulating cholesteric liquid crystal because of its selective reflectibility, and also color display is realized by use of an external single electrode, these display element material has attracted much attentions because of advantages described hereinabove.
On the other hand, amorphous silicon (referred to as a-Si:H hereinafter) that has been proposed as an electronic photographic photosensitive material and has been used practically and organic photosensitive material (referred to as OPC hereinafter) have been known as the element used for a light switching element of a photo-addressable type recording display medium. The photosensitive material such as Se and CdS is not used currently though these are usable for a light switching element, because these are severely harmful for the environment and human body.
a-Si:H has been used practically as photosensitive material for electronic photography and the high sensitivity and high hardness are characteristic of a-Si:H. However, a-Si:H is disadvantageous in that the manufacturing cost is high because a large scale facility is necessary for manufacturing because it is manufactured by chemical vapor deposition and the film depositing speed is as low as several xcexcm/h. As the result, a-Si:H is replaced by OPC because of its low cost and high sensitivity.
A general schematic structure of a photo-addressable type recording display medium provided with such display element and light switching element is shown in FIG. 12. In FIG. 12, the photo-addressable type recording display medium 70 has a laminate structure in which a conductive substrate 60, a charge generation layer (CGL) 62 formed of OPC placed on the conductive substrate 60, and a charge transfer layer (CTL) 64 formed of OPC are laminated in the order from the bottom. The charge generation layer 62 functions as a light switching element layer and generates holes and electrons in the layer when it is irradiated with a light. The charge transfer layer 64 is formed of OPC that functions to transfer electrons, and functions to transfer holes or electrons generated in the charge generation layer 62 in one-way or in two-way fashion.
In the case that an image is to be formed by use of the recording display medium 70 having the structure as described hereinabove, at first ions are charged on the surface of the charge transfer layer 64 side, and the surface is irradiated with a light having the distribution corresponding to the image data. When the surface is irradiated with the light, holes and electrons are generated in quantity corresponding to the light quantity. The one of generated holes and electrons is attracted to the ions charged on the surface of the charge generation layer 62, moves toward the surface side of the charge generation layer 62, and offsets the charges of ions on the surface.
In other words, charges on the part irradiated with the light is extinguished and on the other hand charges on the part not irradiated with the light remains unchanged. Thereafter, when charged toner is supplied on the charge transfer layer 64 side surface, toner remains only on the part where the charge remains, and the toner image that remains on the charge transfer layer 64 side surface is transferred and fixed on a recording medium to thereby obtain an image.
Elgraphy system (referred to as Elgraphy hereinafter) that uses a display medium having the structure as described hereinabove as a recording display medium for an electrostatic image recording system is proposed in Japan Hardcopy, 1996, Fall Meeting, pp. 25.
The Elgraphy is an alternative to development system in photography, and has attracted much attention as a system used for reproducing an image that has been easily taken by a camera at high accuracy. In the Elgraphy, a recording display medium is formed by combining a liquid crystal layer formed of memory liquid crystal and an OPC layer together. The OPC layer, which is used as a light switching element, functions to apply a DC voltage on the recording display medium and to selectively irradiate the recording display medium with a light to thereby generate charges in the OPC layer. The OPC layer transfers the charges to the liquid crystal layer and applies an electric field on the liquid crystal layer to thereby orient liquid crystal molecules, and thus the storable image is displayed.
The Elgraphy is used exclusively for recording mainly and not used for rewriting, but it is possible to rewrite an image. In the case that an image is to be rewritten, the image is erased thermally as described in, for example, Japan Hardcopy, 1996, Fall Meeting, p25, and the recording processing described hereinabove is performed again.
However, the Elgraphy having the structure as described hereinabove is involved in problem in repetitive recording because the AC driving is not effective. In detail, because electron attractive material or electron donating material is used as the charge transportable material of the charge transport layer usually, only any one of electron or hole is transported through the charge transport layer. Therefore, a light switching element having such charge transport layer functions as a rectifier.
Therefore, it is difficult to apply the electric field of one polarity out of the electric fields of the positive and negative polarity, it is substantially equivalent to the application of DC bias on the liquid crystal. As the result, ions in the liquid crystal moves to the place near the electrode by means of action of the bias, the electric field thereby generated near the electrode prevents switching, and no switching causes the image sticking. That is a problem.
To prevent the problem of image sticking due to movement of ions in the liquid crystal, the positive and negative AC electric field is applied usually. Though some charge transportable material such as polyvinyl carbazole is served for bipolar transport, and such material is less sensitive and is not used practically.
The conventional recording principle of such photo-addressable type modulation element or photo-addressable type recording medium is described in, for example, xe2x80x9cO plus E, 1997, No. 206, pp. 115-119xe2x80x9d, and described herein under.
In detail, in a photo-addressable type modulation element or display medium shown in FIG. 13A and FIG. 13B, a display element having a type of threshold value and a light switching element are connected in series and a voltage is applied on both ends. Because the respective capacity components of the display element 80 and the light switching element are approximately constant in respect to the light quantity, and the impedance can be controlled by means of resistance component.
The display is controlled by controlling voltage division ratio of impedance of the display element 80 and light switching element 82.
Particularly, in the case that the impedance of the light switching element layer is lowered, namely in the case that low resistance is used, xe2x80x9cconductive statexe2x80x9d electrical equivalent circuit is used as described in SID 96, APPLICATIONS DIGEST, pp. 59-60.
The term xe2x80x9cconductive statexe2x80x9d described herein means the state in which the resistance component of the light switching element layer is extremely reduced to a low impedance during irradiation with light, or means the state in which the resistance is reduced to an impedance so low as regarded as conductor as an electric equivalent circuit. In the numerical expression, xe2x80x9cconductive statexe2x80x9d means the state that the resistance component of the light switching element layer is reduced to approximately {fraction (1/100)} resistance component of the display element layer.
In this state, the effect of the capacitance component is very small and limited. Thereby, the division voltage becomes high, and as the result the voltage exceeds the threshold voltage of the display element layer and the display is turned on. Because of such structure, it is required for the light switching element to have the resistance component that is reduced to as small value as possible when irradiation is performed.
On the other hand, when the display is to be turned off, high impedance is required, and a light switching element having the resistance component value that becomes low when irradiation with light is performed and becomes high when irradiation with light is not performed is necessary as described in Japanese Published Unexamined Patent Application No. Hei 10-20328.
For example, a display medium in which a-Si:H is used for a light switching element and ferroelectric liquid crystal is used for a display element has been known as described in APPLIED OPTICS pp. 6859-6868.
The resistivity of the resistance component of a-Si:H falls from 1.0xc3x971011 xcexa9cm to 1.0xc3x97108 xcexa9cm with light radiation of 1 mW/cm2. On the other hand, because the resistivity of the display element layer is  greater than 1.0xc3x971010 xcexa9cm, the impedance of the light switching element layer becomes conductive by irradiation with light.
In other words, because the light switching element becomes conductive when irradiation with light is performed, the voltage applied to the display element layer becomes high, and as the result the display is turned on. On the other hand, the impedance of the light switch becomes high during non-irradiation with light, and the voltage applied to the display element becomes low to an amplitude lower than the threshold value of the display element, and as the result the display is turned off.
Therefore, a display medium is designed so that the light switching element becomes conductive and a voltage higher than the threshold value is applied to the display element during irradiation with light and the impedance of the light switching element layer becomes high and a voltage lower than the threshold value is applied to the display element during non-irradiation with light. Thereby, ON/OFF switching of the display unit is controlled. The display unit is switched between ON and OFF by operating the irradiation between ON and OFF selectively.
A photo-addressable type recording display medium in which cholesteric liquid crystal is used for the display element and a-Si:H is used for the light switching element is proposed as a detachable recording display medium provided with a display element formed of the above-mentioned memory display material in, for example, SID 96, APPLICATIONS DIGEST pp. 59-62.
FIG. 14 shows the reflectance characteristic of the photo-addressable type recording display medium to the externally applied voltage. In the case of this photo-addressable type recording display medium, the light switching element layer becomes conductive and the division voltage becomes small, and the threshold value during application of voltage becomes low (namely low threshold value Va) by irradiation with light. On the other hand, the resistance becomes high and the impedance becomes high, and the threshold value becomes high (namely high threshold value Vb) during non-irradiation with light.
Therefore, as shown in FIG. 14, the applied voltage is set to be in the relation Va less than Vc less than Vb, and the irradiation with light is selectively performed for image recording. The division voltage due to fluctuation of the impedance is designed so as to be equal to or higher than the threshold value when the display is ON, on the other hand so as to be lower than the threshold value when the display is OFF.
However, in the case of such recording, because it takes a some long time for the division voltage to approach the voltage division ratio corresponding to the impedance, it is required that the recording pulse is continuously applied until the division voltage approaches the voltage division ratio corresponding to the impedance.
Furthermore, the much quantity of light is needed to render the impedance of the light switching element layer low for light writing, and this is also a problem.
Heretofore, the irradiation with light is performed as much as possible as long as the condition is allowable when the irradiation renders the resistance component low. Particularly, in the case that Bi12SiO20 element or organic photosensitive element is used for the light switching element, the resistance component during non-irradiation with light is very large, and much light quantity is inevitably required to render the resistance of the light switching element layer conductive. For example, the resistance components of organic photosensitive material is equal to or larger than 100 Mxcexa9/cm2 usually, or equal to or larger than 1 Gxcexa9/cm2 in some cases. The irradiation with light of 1 mW/cm2 or more is needed to render the resistance of the light switching element layer conductive 1 Gxcexa9/cm2 or less.
Furthermore, the recording display medium of this type is involved in another problem that the pulse application time is long. In detail, because the time constant of the light switching element or display element layer during non-irradiation with light is long, it is required that a voltage should be applied continuously until the applied voltage is stabilized.
For example, in the case of a photo-addressable type modulation element provided with the above-mentioned cholesteric liquid display element and a-Si:H light switching element that are combined together, because the resistance is approximately 10 Mxcexa9/cm2 and the capacitance is approximately 4 nF/cm2, the time constant is approximately 40 ms resultantly, and approximately 160 ms is required to reach the resistance component ratio of the impedance.
On the other hand, in the case that organic photosensitive material is used for the light switching element, the resistance is approximately 1 Gxcexa9/cm2 and the capacitance is approximately 1 nF/cm2 usually under dark resistance, namely during non-irradiation with light. In this case, the time constant is inevitably as long as 1 second. In addition to the above-mentioned problem, the recording display medium is involved in another problem in the case that liquid crystal material is used for the display element. In detail, ions in the liquid crystal are removed sometimes to improve performance of the display, at that time the resistance component becomes a value equal to or larger than several tens Mxcexa9/cm2 or and the capacitance component becomes a value in a range from 0.1 nF/cm2 to several tens nF/cm2 because of the structure described hereinabove, and the time constant becomes resultantly as long as several hundreds m seconds to 1 second inevitably.
Furthermore, the conventional recording method is involved in another problem that only a positive image is obtained for a writing image. The positive recording means a recording method that the display is turned on brightly during irradiation with light and the display is turned off darkly during non-irradiation with light, and on the other hand the negative recording means a recording method that the display is turned off darkly during non-irradiation with light and the display is turned on brightly during non-irradiation with light. When the display is turned on, the state is in high reflectance in the case of the reflection type and the state is in high light transmittance in the case of the transmission type. When the display is turned off, the state is in low reflectance in the case of the reflection type and the state is in low transmittance in the case of the transmission type.
The present invention has been made in view of the above circumstances and provides a photo-addressable type recording display apparatus that is capable of high sensitivity recording and displaying with a short writing pulse application time. Furthermore, the present invention provides a photo-addressable type recording display apparatus that is capable of switching between positive display and negative display easily.
The photo-addressable type recording display apparatus according to the present invention includes: a photo-addressable type recording display medium provided with a memory display element layer having a predetermined impedance and a light switching element layer having an impedance that is variable depending on irradiation with light laminated on the display element layer and electrically connected in series to the display element layer; a pattern light irradiation source that irradiates the photo-addressable type recording display medium with a pattern light that has been converted corresponding to image information; a pulse voltage application part that applies a predetermined pulse voltage to the photo-addressable type recording display medium; and a driving control part that controls the pulse voltage applied by the pulse voltage application part in accordance with a result of comparison between the predetermined impedance of the display element layer and the impedance of the light switching element layer varied depending on the quantity of light from the pattern light irradiation source for controlling a pulse waveform and a voltage amplitude of the voltage applied to the display element layer, thereby controlling the display state of the photo-addressable type recording display medium.